My long-term research objective is to systematically profile language acquisition in children learning Vietnamese as a first language (L1) and English as a second language (L2). A clear understanding of interactions between L1 and L2 in developing bilinguals will contribute to unified theories of language that account for language acquisition under diverse circumstances, including children learning two typologically distinct languages (such as Vietnamese and English), beginning at two different points in their social and cognitive development. Currently there are no available studies on Vietnamese language development. As an initial step in the research process, the proposed study focuses on lexical semantic development of 30 early elementary school-aged children learning Vietnamese (L1) and English (L2). Eight different lexicalsemantic measures (2 types [knowledge and processing-based] x 2 modalities [receptive and expressive] x 2 languages [Vietnamese and English]) will be administered to each participant at three different time points, with a one-year interval between test times. There are two specific aims. First, Hierarchical Linear Modeling will be used to analyze growth patterns of L1 and L2. Second, correlation and regression analyses will be used to investigate potential relationships within and across languages at each time point as well as across time. The current proposal may well be the first longitudinal investigation of bilingual lexical semantic development using both knowledge-based and processing-based measures. Upon completion of this project, we will better understand the lexical semantic growth trajectories of sequential bilingual children as well as the relationships within each language and across languages over time. In addition, this line of research has clinical applications in creating educational materials for Vietnamese populations at risk for poor language and academic outcomes. PUBLIC HEALTH RELEVANCE: Understanding and supporting the language development of non-English home languages in the U.S. promotes public health on individual and community levels. Strong connections in the home and surrounding community increase social and emotional development. A clear understanding of L1 and L2 development is essential to developing effective early education programs to promote L1 maintenance and pave the way for greater academic and vocational achievements in English, the majority community language.